1. Field of the Invention
The present invention relates to an acceleration power supply used, for example, in a neutral beam injector in a nuclear fusion reactor.
2. Discussion of Background
An acceleration power supply generates a very high DC voltage. Such a high DC voltage is used to accelerate ions in a plasma so that the neutralized ions are injected into fusion plasma etc. An ion source which is a load of the acceleration power supply is frequently short-circuited. The acceleration power supply should therefore have a function to detect such a short-circuit and to cut off at high speed the voltage supply to the load. A main configuration of a conventional acceleration power supply may be as shown in FIG. 1. Referring to FIG. 1, reference numeral 11 denotes a thyristor switch; 12, a transformer; 13, a rectifier; and 14, a filtering capacitor. A power tube 15 used for a series regulator applies a constantly-regulated voltage to a load 18 and, in an emergency, is cut off for safety. In order to restrict the plate dissipation of power tube 15 below the rated value, the triggering angle of thyristor switch 11 is so controlled that a voltage on capacitor 14 is always higher by a prescribed voltage than a load voltage applied to load 18. Then, the voltage drop across power tube 15 is substantially fixed.
The load voltage is controlled in the following manner. The control grid of power tube 15 is coupled via a power tube drive circuit 16 to a voltage control circuit 20. A control signal E20 from voltage control circuit 20 serves to control the power tube such that an output voltage signal E17 obtained from a voltage divider 17 becomes equal to a reference voltage signal E19 from a presetter 19. Output voltage signal E17 is proportional to the load voltage and, therefore, the load voltage is automatically follows the value of reference voltage signal E19. When control signal E20 is supplied to power tube drive circuit 16 and the grid voltage of power tube 15 is set at a prescribed value, a given constant load voltage is obtained. In order to apply a stable voltage to load 18, a high-speed control operation for power tube 15 is required.
Incidentally, power tube 15 is also used to quickly cut off an over current to load 18 when a short-circuit of load 18 is detected.
Generally speaking, the frequency response of power tube 15 extends beyond several megahertz or more. For this reason, when the response speed of each of voltage control circuit 20 and power tube drive circuit 16 is designed to be fast, the total response speed of the automatic voltage control loop is substantially determined by the time constant of load 18. This time constant is generally formed of a capacitance component and resistance component of load 18 (not shown).
Thyristor switch 11 also has a function to control the voltage on capacitor 14. However, the response speed of thyristor switch 11 is much lower than that of power tube 15. In addition, thyristor switch 11 does not have a function to positively decrease the voltage on capacitor 14. This fact requires a high-speed control operation for power tube 15 in order that a stable output voltage is supplied to load 18.
Typical ratings of an acceleration power supply for a neutral beam injector for heating a plasma is, for instance,:
output voltage=100 kV; PA1 output current =100A; PA1 rise time=several hundred microseconds; and PA1 cutoff time=about 20 microseconds. PA1 K. A. Milines et al. PA1 Lawrence Berkeley Laboratory PA1 University of California PA1 August 1978; and PA1 J. V. Franck et al. PA1 Lawrence Berkeley Laboratory PA1 October 1977.
The above ratings can be satisfied by the conventional system as shown in FIG. 1. However, in recent years, voltage and current ratings of acceleration power supply have been increasing and the above ratings are now insufficient in many cases. In view of this, it is necessary to connect a given number of power tubes in series and/or in parallel, or to use a special power tube having large voltage and current ratings, in order to satisfy the increased ratings. However, if the conventional system as shown in FIG. 1 is used in such cases, the following problems are invited.
A power tube having large voltage and current ratings is very expensive, has a very short filament life (generally below 3,000 hours), and tends to cause a flashover (short-circuit) within the power tube so that the reliability of the power supply becomes low. Further, when the working voltage is exceeds over around 10 kV, X-rays are generated and an X-ray shielding member being formed of a lead plate must be provided.